In today's wireless networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), and Ultra Mobile Broadband (UMB). A wireless network comprises wireless base stations which provide wireless coverage over a geographical area, typically called a cell.
There is a need to synchronise operation of base stations. Synchronisation allows a base station to generate the correct RF carrier frequency and to time transmission of a wireless signal on the wireless interface. Depending on the type of technology, adjacent cells may operate at the same frequency or at different frequencies. A base station should operate at a required frequency, and should not drift from that frequency. A typical requirement for frequency accuracy is around 50 parts per billion (ppb). When base stations use Time Division Multiplexing (TDM) or Time Division Duplexing (TDD) there is also a requirement for the base stations to have access to an accurate time/phase synchronisation reference, in addition to frequency synchronisation. Time/phase synchronisation is required for the correct generation of the TDD frame on the radio interface to avoid interference between the signals generated by adjacent cells. A typical requirement for time/phase synchronisation is of the order of +/−1.5 μs.
As an example, indoor coverage problems and increasing demand for data in both indoor and outdoor locations are two of the biggest issues related to current mobile network planning, especially with the migration to 4G, due to its higher frequency bands that cause even more path and penetration losses and shorten coverage range. Operators can use small cells (such as micro cells or pico cells) to expand capacity and fill coverage holes in their networks cost-effectively. The evolution of mobile networks is presenting an increased demand to coordinate radio base stations and therefore also requiring the distribution of a phase/time sync reference.
In some base station implementations a radio unit and a baseband processing equipment (also called a digital unit (DU)) are combined. In other implementations, the radio unit and DU are separated and can be split between two different locations. In this case, the radio unit is called a remote radio unit (RRU). The radio unit creates an analog transmit RF signal from the baseband signal and provides the RF signal to an antenna. The radio unit correspondingly digitises an RF receive signal. The DU and RRU are connected via e.g. an optical network. The one or more DUs may be centralised and located remotely, for example a few kilometres from the RRUs. The RRUs are placed close to the radio antennas, e.g. in antenna masts. This minimises feeder and jumper losses between antenna and RRUs, which is often a major challenge to address in most radio transport networks, for example, to enhance the uplink capacity of mobile services. The signal processing is centralised in a DU, which offers processing resources for multiple cells, each covered by an antenna driven by a RRU. This allows a pool of processing resources to be dynamically shared among many cells, in line with the cloud computing principle, saving energy, improving the radio link reliability and decreasing number and size of access sites. A Common Public Radio Interface (CPRI) specifies a Time Division Multiplexing (TDM) protocol for carrying data between DUs and RRUs.
A wireless network may comprise a mix of base station types, such as (i) one or more base stations with a split RRU and DU, and (ii) one or more monolithic base stations with RF and baseband processing at the base station. It is desirable that base stations of different types are synchronised in frequency and/or time.